Run tests in Appveyor builds

[madmaxoft]

Currently, tests are not run in Appveyor CI. I almost managed to enable them, but ran into an issue - three of the tests failed to run because they didn't find some DLL:
https://ci.appveyor.com/project/madmaxoft/mcserver/builds/26969702#L1389

11/26 Test #11: FastRandom-test ....................   Passed    0.05 sec
      Start 12: LoadablePieces-test
12/26 Test #12: LoadablePieces-test ................Exit code 0xc0000135
***Exception:   0.03 sec
      Start 13: PieceRotation-test
13/26 Test #13: PieceRotation-test .................   Passed    0.02 sec
      Start 14: PieceGeneratorBFSTree-test
14/26 Test #14: PieceGeneratorBFSTree-test .........Exit code 0xc0000135
***Exception:   0.02 sec
      Start 15: HTTPMessageParser_file-test1-2
15/26 Test #15: HTTPMessageParser_file-test1-2 .....   Passed    0.02 sec
      Start 16: HTTPMessageParser_file-test1-128
16/26 Test #16: HTTPMessageParser_file-test1-128 ...   Passed    0.01 sec
      Start 17: HTTPMessageParser_file-test2
17/26 Test #17: HTTPMessageParser_file-test2 .......   Passed    0.01 sec
      Start 18: HTTPMessageParser_file-test3-2
18/26 Test #18: HTTPMessageParser_file-test3-2 .....   Passed    0.01 sec
      Start 19: HTTPMessageParser_file-test4-512
19/26 Test #19: HTTPMessageParser_file-test4-512 ...   Passed    0.01 sec
      Start 20: UrlClient-test
20/26 Test #20: UrlClient-test .....................   Passed    1.05 sec
      Start 21: LuaThreadStress-test
21/26 Test #21: LuaThreadStress-test ...............Exit code 0xc0000135
***Exception:   0.01 sec

(Exit code 0xc0000135 means "DLL not found").

Appveyor is especially well suited to run tests, because it builds a Debug version of everything, so it includes the assert-checking tests.